Chlorophenol binding to dissolved and particulate soil organic matter determined in controlled equilibrium systems

Summary We determined the sorption of 2,4‐dichlorophenol (DCP), 2,4,5‐trichlorophenol (TCP) and pentachlorophenol (PCP) to dissolved (DOM) and particulate soil organic matter (POM) from the same soil in controlled equilibrium systems, using 14C‐labelled chlorophenols in combination with reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and liquid scintillation. Associations of DCP, TCP and PCP to DOM and POM were satisfactorily described by linear adsorption isotherms. Together with the absence of substantial competition between DCP and TCP for binding sites, this indicates a hydrophobic partitioning mechanism. The organic carbon normalized partitioning coefficient (KOC) for the binding of DCP was similar in magnitude for POM (KPOC) and for DOM (KDOC), whereas KPOC for the more hydrophobic compounds TCP and PCP were approximately one order of magnitude greater than KDOC. On the basis of the relationships between log KOC and the organic carbon normalized partitioning coefficient (log KOW), the extent of association to POM increases more with the hydrophobicity of the chlorophenol than the extent of association to DOM. This holds for our data obtained for DOM and POM of similar origin, as well as for various sources of POM and DOM reported in the literature. Differences in the magnitude of KPOC and KDOC in our study could not be accounted for by differences in gross carbon chemistry of POM and DOM, as determined by nuclear magnetic resonance (13C‐NMR) and X‐ray photoelectron spectroscopy (XPS). Thus, other factors such as the average size and capacity of hydrophobic moieties could explain differences in chlorophenol association between POM and DOM. We conclude that KPOC and KDOC need to be determined explicitly, when the transport and retention of chlorophenols is modelled, and not calculated from relationships between log KOC and log KOW.